Automotive parts, especially parts used for a transmission such as CVT pulley shafts, are generally manufactured by surface hardening treatment such as carburizing and quenching followed by tempering, from the viewpoint of improving the bending fatigue strength and wear resistance.
In general, the “carburizing and quenching” is a treatment in which using a low-carbon “case hardening steel” as a raw material steel (base metal steel), and C has been intruded and diffused in the austenitic region at a high temperature of Ac3 point or higher, then the steel is quenched.
In recent years, motor vehicles are required to have a lighter weight and higher torque. To meet this requirement, the carburized parts such as the CVT pulley shafts must have a higher bending fatigue strength and higher wear resistance than before. In this description, hereinafter an explanation may be given by referring to the “CVT pulley shaft” as a representative of the “carburized part”.
When large amounts of alloying elements such as Ni, Cr and Mo are added to a case hardening steel, the CVT pulley shaft can exhibit a high bending fatigue strength and high wear resistance; however, the component cost is increased by the increased amount of alloying elements.
However, both of Ni and Mo are important elements that increase the depth of a carburized layer and the hardness of a core part (base metal), and also are elements for improving the temper softening resistance. Moreover, both of Ni and Mo also have an effect of improving the hardenability of carburized layer without increasing the depth of an intergranular oxidation layer formed on the surface during gas carburization because these elements are nonoxidizing elements.
Therefore, as a “case hardening steel”, which serves as a raw material for CVT pulley shaft, a “chromium-molybdenum steel” such as SCM420H defined in JIS G 4052 (2008) is often used. However, in view of the situation of a recent steep rise in Mo cost, there is a greatly increasing demand for a case hardening steel material in which the addition amount of Mo is kept as small as possible, whereby the component cost is decreased, and moreover, the CVT pulley shaft can be provided with a high bending fatigue strength and high wear resistance.
Accordingly, to meet the above-described demand, for example, Patent Documents 1 and 2 propose a “high chromium steel for carburizing and carbo-nitriding treatment” and “method for manufacturing case-hardened product having high fatigue strength”, respectively.
Specifically, Patent Document 1 discloses a “high chromium steel for carburizing and carbo-nitriding treatment” obtained by heating a steel consisting of, by mass percent, C: 0.10 to 0.30%, Si: 0.15% or less, Mn: 0.90 to 1.40%, P: 0.015% or less, Cr: 1.25 to 1.70%, Al: 0.010 to 0.050%, Nb: 0.001 to 0.050%, O: 0.0015% or less, and N: 0.0100 to 0.0200%, further containing, as necessary, one or more kinds of elements selected from (a) Ni: 0.15% or less and Mo: 0.10% or less, (b) Ti: 0.005 to 0.015%, and (c) S: 0.005 to 0.035%, Pb: 0.01 to 0.09%, Bi: 0.04 to 0.20%, Te: 0.002 to 0.050%, Zr: 0.01 to 0.20%, and Ca: 0.0001 to 0.0100%, and the balance being Fe and unavoidable impurity elements, to 1200° C. or higher, finishing hot forming such as hot rolling at a finishing temperature of 800° C. or higher, and thereafter cooling the steel to 600° C. or lower at an average cooling rate of 30° C./min or higher.
Also, Patent Document 2 discloses “method for manufacturing case-hardened product having high fatigue strength”, wherein a steel material consisting of, by mass ratio, C: 0.10 to 0.30%, Mn: 0.50 to 2.0%, S: 0.01 to 0.20%, Cr: 0.50 to 1.50%, Al: 0.02 to 0.10%, and N: 0.010 to 0.025%, it being restricted such that Si: 0.10% or less, P: 0.010% or less, and O: 0.005% or less, further containing, as necessary, one or more kinds of elements selected from (a) Nb: 0.020 to 0.120% and Ti: 0.005 to 0.10%, and (b) Ni: 4.0% or less, Mo: 1.0% or less, V: 1.0% or less, and Cu: 3.0% or less, and the balance being Fe and unavoidable impurities, is worked into a required product shape and is subjected to carburizing treatment under the condition that the amount of retained austenite in a 0.02-mm outer layer is in the range of 20 to 60% in area fraction, and thereafter repeated bending stresses in the range of 70 to 120 kgf/mm2 (686 to 1176 MPa) in the net maximum stress on the outermost surface are applied 103 times or less to a stress concentrating part.